docs: def add_unqualified_edge(self, u: BaseEntity, v: BaseEntity, relation: str) -> str:
          """Add a unique edge that has no annotations.
  
          :param u: The source node
          :param v: The target node
          :param relation: A relationship label from :mod:`pybel.constants`
          :return: The key for this edge (a unique hash)
          """
          attr = {RELATION: relation}
          return self._help_add_edge(u, v, attr)